Electrochromic materials and devices have been developed as an alternative to passive coating materials for light and heat management in building and vehicle windows. In contrast to passive coating materials, electrochromic devices employ materials capable of reversibly altering their optical properties following electrochemical oxidation and reduction in response to an applied potential. The optical modulation is the result of the simultaneous insertion and extraction of electrons and charge compensating ions in the electrochemical material lattice.
In general, electrochromic devices have a composite structure through which the transmittance of light can be modulated. FIGS. 1A and 1B illustrate plan and cross-sectional views, respectively, of a typical prior art electrochromic device 20. The device 20 includes isolated transparent conductive layer regions 26A and 26B that have been formed on a substrate 34, such as glass. In addition, the device 20 includes a counter electrode layer 28, an ion conductive layer 32, an electrochromic layer 30 and a transparent conductive layer 24, which have been deposited in sequence over the conductive layer regions 26. It is to be understood that the relative positions of the electrochromic and counter electrode layers of the device 20 may be interchanged. Further, the device 20 includes a bus bar 40 which is in contact only with the conductive layer region 26A, and a bus bar 42 which may be formed on the conductive layer region 26B and is in contact with the conductive layer 24. The conductive layer region 26A is physically isolated from the conductive layer region 26B and the bus bar 42, and the conductive layer 24 is physically isolated from the bus bar 40. Although an electrochromic device may have a variety of shapes, such as including curved sides, the illustrative, exemplary device 20 is a rectangular device with the bus bars 40 and 42 extending parallel to each other, adjacent to respective opposing sides 25, 27 of the device 20, and separated from each other by a distance W. Further, the bus bars 40 and 42 are connected by wires to positive and negative terminals, respectively, of a low voltage electrical source 22 (the wires and the source 22 together constituting an “external circuit”).
Referring to FIGS. 1A and 1B, when the source 22 is operated to apply an electrical potential across the bus bars 40, 42, electrons, and thus a current, flows from the bus bar 42, across the transparent conductive layer 24 and into the electrochromic layer 30. In addition, if the ion conductive layer 32 is an imperfect electronic insulator as is the case in many thin film electrochromic devices, a small current, commonly referred to as a leakage current, flows from the bus bar 42, through the conductive layer 24 and the electrochromic layer 30, and into the ion conductive layer 32. Further, ions flow from the counter electrode layer 28, through the ion conductive layer 32, and to the electrochromic layer 30, and a charge balance is maintained by electrons being extracted from the counter electrode layer 28, and then being inserted into the electrochromic layer 30 via the external circuit. As the current flows away from the bus bar 42 across the conductive layer 24 and towards the bus bar 40, voltage is dropped by virtue of the finite sheet resistance of the conductive layer 24, which is typically about 3-20 Ohms/square. In addition, current flowing across the conductive layer 24 is incrementally reduced, as current is drawn through the combination of the layers 30, 32 and 28 (“stack”) to produce the electrochromic coloration in the device 20.